1. Field of the Invention
The present invention relates to a transmitter/ receiver system for data communications.
2. Description of the Prior Art
Transmitter/receiver systems employing a multi-frequency signal have heretofor employed a method of frequency-modulating a carrier with the multi-frequency signal for transmission. This method is advantageous in that any frequency drift of the carrier due for example to temperature does not affect the frequency of the multi-frequency signal. However, the method has the following two drawbacks:
(1) Since the carrier is frequency-modulated by the multi-frequency signal, the bandwidth in a transmission line is increased as compared with that of the multi-frequency signal.
(2) A demodulator circuit for demodulating the frequency-modulated signal suffers a problem. More specifically, when the demodulator circuit receives a signal having an S/N ratio lower than a threshold level for improving the S/N ratio, the demodulator circuit produces a demodulated output having quite a bad S/N ratio. Such a phonomenon is caused in principle by any FM demodulator circuits. When a weak signal having an S/N ratio lower than the threshold level, therefore, the demodulated multi-frequency signal has an extremely bad S/N ratio and cannot properly be detected. Accordingly, a limit signal level that can be recieved depends on the threshold level for improving the S/N ratio in the FM demodulator circuit, and there is a limitation on the distance for which signals can be transmitted and received.